As discussed at the hacking day of Contributor Summit.

Will serve as the basis for asynchronous file I/O (ie QTBUG-136764) => all backends need to be asynchronous.

Not based on QIODevice, and will provide vectored read/writes (aka scatter/gather IO), the read/write requests are fulfilled fully asynchronously, using native APIs where available.

Random notes:

• Implement using io_uring on Linux
• Look into ioringapi from Windows
  • Otherwise completion ports or overlapped io
• Potentially use grand central dispatch on macOS, but better if it also works on other BSDs
• QThreadPool based fallback for platforms with insufficient API or missing backend

To be implemented as an internal detail at first before we cement it and make it public.